linux-kernel - Re: [RFC PATCH V2 0/3] sched/fair: Fix nr-running vs delayed-dequeue

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Message-ID: <CAKfTPtBahSErKvd5y698v7+EZYGkT95dbO0LumZAWD=ut1vj3Q@mail.gmail.com>
Date: Wed, 5 Mar 2025 09:17:28 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, ke.wang@...soc.com, di.shen@...soc.com
Subject: Re: [RFC PATCH V2 0/3] sched/fair: Fix nr-running vs delayed-dequeue

On Tue, 4 Mar 2025 at 02:56, Xuewen Yan <xuewen.yan94@...il.com> wrote:
>
> Hi Peter
>
> On Mon, Mar 3, 2025 at 8:00 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Mon, Mar 03, 2025 at 06:52:38PM +0800, Xuewen Yan wrote:
> > > Delayed dequeued feature keeps a sleeping sched_entitiy enqueued until its
> > > lag has elapsed. As a result, it stays also visible in rq->nr_running.
> > > However, sometimes when using nr-running, we should not consider
> > > sched-delayed tasks.
> > > This serie fixes those by adding a helper function which return the
> > > number of sched-delayed tasks. And when we should get the real runnable
> > > tasks, we sub the nr-delayed tasks.
> > >
> >
> > Is there an actual performance improvement? Because when a runqueue
> > looses competition, delayed tasks very quickly dissipate.
>
> At the moment, I don't have very detailed test data. I've been
> studying delay-dequeue carefully recently, and these are the issues I
> feel might need modification as I go through the code.

Patch 1 makes sense for me but I'm less convinced by patch 2 and 3. As
Peter also mentioned, the state where cpu_rq(cpu)->nr_running ==
cfs_h_nr_delayed(cpu_rq(cpu)) is really transient as they will be
picked as soon as the last runnable task will be dequeued

>
> Thanks!
>
> BR